1. Field of the Invention
The present invention relates to an optical recording medium, such as an optical disk, and a method of manufacturing the same, and more particularly, to an optical recording medium in which address information is recorded by wobbling pregrooves.
2. Description of the Related Art
In order to record data onto a disk, address information must be recorded so that data can be recorded at predetermined positions. In one such case, this address information is recorded by wobbling.
That is, a track for recording data is formed in advance, for example, as a pregroove. The side wall of this pregroove is wobbled (meandered) in accordance with address information.
When this is done, an address can be read from the wobbling information. Even if, for example, pit data and the like indicating an address is not formed on a track in advance, it is possible to record and reproduce data at a desired position.
FIGS. 1A and 1B show an example of the structure of a groove of an optical disk. As shown in FIG. 1A, the track on the disk 100 is formed as a pregroove 101 in advance in a spiral shape from the inner region toward the outer region.
At parts of this pregroove 101, as shown in exploded view in FIG. 1B, the side walls on the right and left are wobbled in accordance with address information. That is, they are meandered at predetermined cycles in accordance with a wobbling signal generated on the basis of the address. Between the pregroove 101 and a pregroove 101 adjacent thereto is a land 102, and, for example, recording of data is performed in the pregroove 101.
In such a wobbled pregroove 101, for example, not only address information, but also information as a fine clock mark used for clock synchronization can be contained.
As wobbled grooves, by disposing wobbled portions in accordance with this fine clock mark at predetermined intervals, the reproduced information can be, for example, fine radius position information in one circular track on the disk.
An operation for generating a fine clock mark and detecting the fine clock mark will now be described with reference to FIG. 2.
In a step for manufacturing a disk (forming tracks by wobbled pregrooves), a fine clock mark signal of a rectangular wave having a high frequency as in FIG. 2(a) is generated at predetermined intervals, for example, as absolute address data.
This is synthesized with a signal such that an absolute address is modulated by a predetermined carrier, generating a wobble signal such as that of FIG. 2(b).
In an operation for cutting a master disk, by deflecting a laser beam for exposure in accordance with a wobble signal, the exposure track is wobbled. In the disk manufactured from such a master disk, a wobbled groove which meanders in accordance with an address and a fine clock mark as in FIG. 2(c) is formed.
In a recording and reproduction apparatus, when a laser spot LS is radiated onto a track (groove 101) as in FIG. 1B, of the groove reproduced information obtained as reflected light therefrom, a signal FCM for use in fine clock mark detection is as shown in FIG. 2(d). In an operation for detecting a fine clock mark, by detecting the zero-cross edge of the signal FCM, a timing tCM is made to be a fine clock mark timing.
However, the groove is made to meander in its entirety. Therefore, since a zero-cross edge is detected at other than the timing tCM as in FIG. 2(e), a window is generated to mask an unwanted zero-cross edge.
In order to generate a window, a signal FCM is differentiated to obtain a differential signal of FIG. 2(f), and this is sliced by predetermined threshold values TH1 and TH2. That is, to form a window for taking out only the section (section in which the timing tCM is the center) in which there are sharp amplitude variations as a fine clock mark, the threshold values TH1 and TH2 are generated, and a window such as that in FIG. 2(g) is generated.
Here, in the example of FIG. 2(g), an example is shown in which a window is correctly generated for a window W11. That is, as can be seen from a comparison with the zero-cross edge signal of FIG. 2(e), the window W11 is a window such that only the zero-cross edge at timing tCM can be extracted.
However, since the differential value becomes high even at the leading portion and the trailing portion of the amplitude as a fine clock mark, some degree of amplitude of the differential signal appears even at the leading portion and the trailing portion thereof as in FIG. 2(f).
Since the amplitude of a window W10 at the leading portion and the trailing portion thereof exceeds the threshold value TH1, it has a wider window width than is required.
In this case, when the zero-cross edge signal is extracted in the window W10, zero-cross edge detection is performed at other than the original fine clock timing tCM, presenting the problem that a fine clock mark timing cannot be detected accurately.
That is, for a signal FCM obtained in response to a fine clock mark, a large amplitude is produced at other than the original fine clock mark timing as a differential signal thereof, and as a result, a window cannot be generated correctly, and the accuracy of detection of a fine clock mark timing is a problem.